Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
From:   Joel Fernandes <joel@joelfernandes.org>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH rcu/dev 1/3] net: Use call_rcu_flush() for qdisc_free_cb
Date:   Thu, 17 Nov 2022 16:58:26 -0500
Message-Id: <DCDBA54C-C35B-497D-BB39-224C88B94660@joelfernandes.org>
References: <CANn89iJuy=PuAiwrjF3qZY0M+86eRQ=o_x-m-eoxOdyAM8yoSg@mail.gmail.com>
Cc:     linux-kernel@vger.kernel.org, Cong Wang <xiyou.wangcong@gmail.com>,
        David Ahern <dsahern@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Jiri Pirko <jiri@resnulli.us>, netdev@vger.kernel.org,
        Paolo Abeni <pabeni@redhat.com>, rcu@vger.kernel.org,
        rostedt@goodmis.org, paulmck@kernel.org, fweisbec@gmail.com
In-Reply-To: <CANn89iJuy=PuAiwrjF3qZY0M+86eRQ=o_x-m-eoxOdyAM8yoSg@mail.gmail.com>
To:     Eric Dumazet <edumazet@google.com>
Precedence: bulk


> On Nov 17, 2022, at 4:44 PM, Eric Dumazet <edumazet@google.com> wrote:
>=20
> =EF=BB=BFOn Wed, Nov 16, 2022 at 7:16 PM Joel Fernandes (Google)
> <joel@joelfernandes.org> wrote:
>>=20
>> In a networking test on ChromeOS, we find that using the new CONFIG_RCU_L=
AZY
>> causes a networking test to fail in the teardown phase.
>>=20
>> The failure happens during: ip netns del <name>
>>=20
>> Using ftrace, I found the callbacks it was queuing which this series fixe=
s. Use
>> call_rcu_flush() to revert to the old behavior. With that, the test passe=
s.
>>=20
>> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>> ---
>> net/sched/sch_generic.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>=20
>> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
>> index a9aadc4e6858..63fbf640d3b2 100644
>> --- a/net/sched/sch_generic.c
>> +++ b/net/sched/sch_generic.c
>> @@ -1067,7 +1067,7 @@ static void qdisc_destroy(struct Qdisc *qdisc)
>>=20
>>        trace_qdisc_destroy(qdisc);
>>=20
>> -       call_rcu(&qdisc->rcu, qdisc_free_cb);
>> +       call_rcu_flush(&qdisc->rcu, qdisc_free_cb);
>> }
>=20
> I took a look at this one.
>=20
> qdisc_free_cb() is essentially freeing : Some per-cpu memory, and the
> 'struct Qdisc'
>=20
> I do not see why we need to force a flush for this (small ?) piece of memo=
ry.

I=E2=80=99ll try to drop that and rerun the test, and get back to you. It co=
uld be that there is a different callback that this flush() is compensating f=
or, or something. I am pretty sure at one point, dropping this patch made th=
e test fail most of the time. Now it passes 100%.

I=E2=80=99ll also attempt to collect a complete trace, maybe I=E2=80=99ll le=
arn some networking code in the process..

Thanks!=